Optical illusions
An optical illusion is characterized by visually perceived images that, at least in common sense terms, are deceptive or misleading. Therefore, the information gathered by the eye is processed by the brain to give, on the face of it, a percept that does not tally with a physical measurement of the stimulus source. A conventional assumption is that there are physiological illusions that occur naturally and cognitive illusions that can be demonstrated by specific visual tricks that say something more basic about how human perceptual systems work. Physiological illusions Physiological illusions, such as the afterimages following bright lights or adapting stimuli of excessively longer alternating patterns (contingent perceptual aftereffect), are presumed to be the effects on the eyes or brain of excessive stimulation of a specific type - brightness, tilt, color, movement, and so on. The theory is that stimuli have individual dedicated neural paths in the early stages of visual processing, and that repetitive stimulation of only one or a few channels causes a physiological imbalance that alters perception. Example movie which produces distortion illusion after you watch it and look away. Cognitive illusions Cognitive illusions are assumed to arise by interaction with assumptions about the world, leading to "unconscious inferences", an idea first suggested in the 19th century by Hermann Helmholtz. Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions. (a). Ambiguous illusions are pictures or objects that elicit a perceptual 'switch' between the alternative interpretations. The Necker cube is a well known example; another instance is the Rubin vase. (b). Distorting illusions are characterized by distortions of size, length, or curvature. A striking example is the Café wall illusion. Another example is the famous Mueller-Lyer illusion. ©. Paradox illusions are generated by objects that are paradoxical or impossible, such as the Penrose triangle or impossible staircases seen, for example, in M. C. Escher's Ascending and Descending and Waterfall. The triangle is an illusion dependent on a cognitive misunderstanding that adjacent edges must join. (d). Fictional illusions are defined as the perception of objects that are genuinely not there to all but a single observer, such as those induced by schizophrenia or a hallucinogenic substance. These are more properly called hallucinations. Well-known illusions *Afterimage illusion *Ambiguous image *Ames room illusion *Ames Trapezoid Window illusion *Barberpole illusion *Benham's top *Bezold Effect *Blivet (also known as the Impossible trident illusion) *Cafe wall illusion *Chubb illusion *Cornsweet illusion *Delboeuf illusion *Ebbinghaus illusion *Ehrenstein illusion *Ehrenstein's brightness illusion *Ehrenstein's square illusion *Ehrenstein's brightness illusion *Flash lag illusion *Fraser spiral illusion *Grid illusion *Hering illusion *Hermann grid illusion *Hollow-Face illusion *Impossible cube *Isometric illusion *Jastrow illusion *Kanizsa triangle *Lilac chaser *Mach bands *Missing square puzzle *Moon illusion *Motion illusion ** anomalous motion illusion ** Relative motion illusion *Motion induced blindness *Muller-Lyer distortion illusion *Necker cube illusion *Neon color spreading *Orbison illusion *Penrose triangle aka Impossible triangle illusion *Peripheral drift illusion *Phi phenomenon *Poggendorff illusion *Ponzo illusion *Pulfrich effect or Pulfrich pendulum illusion *Rubin vase *Same color *Sander illusion *Size-weight illusion *Vertical–horizontal illusion *Watercolor illusion *White's illusion *Wundt illusion *Zollner illusion Many famous artists have worked extensively with optical illusions, including M.C. Escher, Salvador Dalí, Giuseppe Arcimboldo, Marcel Duchamp, Oscar Reutersvärd, and Charles Allan Gilbert.Optical illusion is also used in film by the technique of forced perspective. Gallery Image:Illusion_movie.ogg| Stare at the centre an example movie which produces distortion illusion after watching the movie then looking away. Image:Glass_tesseract_animation.gif| Tesseract Image:Café wall.svg| Café wall illusion Image:Lilac-Chaser.gif| Lilac chaser See also * Adaptation (eye) * Auditory illusion * Camouflage * Chubb illusion * Contingent perceptual aftereffect * Contour rivalry * Craik-O'Brien-Cornsweet illusion * Depth perception * Emmert's law * Entoptic phenomenon * Fata Morgana * Forced perspective - application used in film and architecture to create the illusion of larger, more distant objects. * Geometrical-optical illusions * Gravity hill * Hermann von Helmholtz * Hollow-Face illusion * Horace Barlow * Impossible object * Kinetic depth effect * List of optical illusions * Mach bands * McCollough effect * Multistable Perception * Op Art * Perception * Perceptual aftereffect * Perceptual distortion * Perceptual disturbances * Spatial distortion * Trompe l'oeil * Unconscious inference * Visual reorientation illusions * Wagon-wheel effect References & Bibliography Key texts Books *Purves D, Lotto B (2002) Why We See What We Do: An Empirical Theory of Vision. Sunderland, MA: Sinauer Associates. Papers *Eagleman, D.M. (2001) Visual Illusions and Neurobiology. Nature Reviews Neuroscience. 2(12): 920-6. (pdf) *Gregory Richard (1997) Knowledge in perception and illusion. Phil. Trans. R. Soc. Lond. B 352:1121-1128 (pdf) *Purves D, Lotto RB, Nundy S (2002) Why We See What We Do. American Scientist 90 (3): 236-242. *Purves D, Williams MS, Nundy S, Lotto RB (2004) Perceiving the intensity of light. Psychological Rev. Vol. 111: 142-158. *Yang Z, Purves D (2003) A statistical explanation of visual space.''Nature Neurosci 6: 632-640. Additional material Books *A. Seckel, The Art of Optical Illusions, Carlton Books, 2000 Papers *Google Scholar *Cooper, A. C. G. & Humphreys, G. W. A New Object-Based Visual Illusion. Psychonomic Bulletin & Review. External links * ooh my brain optical illusions * mighty optical illusions * Mighty Illusions Blog: Still Growing * Ophtasurf : selection of the best optical illusions in the world * Illusion World * Optical illusion - 526 illusions * Theory of Optical Illusions * Optical Illusions & Other Eye Tricks * Anamorphosis 3D multiple * [http://www.peinturemurale.com/ Murals (400 pictures) and a section on '''anamorphosis'] * Illusions by Akiyoshi Kitaoka * Optical Illusions & Visual Phenomena * Visual Paradoxes * Visual Illusions: 30 Java applets help see why they happen * Optical Illusions: Can you believe your eyes? * Large Collection of Optical Illusions * Optical Illusions: Computer Art in the Classroom * Optical Illusions, Online Illusions * Optical Illusions Pictures & Magic Eye Tricks * Grand Illusions * Explanation of many visual illusions and other gestalt effects * Optical Illusions, and Optical Illusions in real life * Color Optical Illusions * Illusion Pages (100+ illusions) * http://www.purveslab.net/main/ Category:Optical illusions Category:Optical phenomena Category:Perception